This invention relates to protecting a moisture-preamble waterproof coated fabric from ultraviolet light deterioration as manifested by yellowing, loss or change of color, or loss of desirable mechanical properties. More particularly, it is concerned with a moisture permeable waterproof fabric having a microporous polymeric coating resistant to degradation caused by light. The coated fabric retains good moisture-permeability with durable waterproofness and other desirable properties that remain characteristic of the fabric even following multiple launderings. Procedures for making such fabrics are also described.
Coated fabrics suitable for use as activewear, rainwear and tentage function by blocking the pores of a woven, knitted or non-woven fabric with a cohesive polymer film which acts as a physical barrier against wind, water, and in the case of protective workwear, aggressive chemicals, oils, and greases. This barrier or coating distinguishes polymer coatings from chemical finishes which merely coat the individual fibers of a fabric without blocking the pores, and repel fluids by surface tension effects. Microporous coated fabrics repel water from the outside yet allow perspiration and moisture vapor to escape from the inside. Moisture is transmitted through a tortuous physical pathway produced in the cellular film or coating resulting from the art-recognized wet coagulation process, as described in more detail below. Polymeric coatings have initially been based upon rubber or synthetic or fluorocarbon rubbers, and more recently, polyurethanes, acrylics, silicone elastomers and polyvinylchlorides.
Fashion and leisurewear, particularly rainwear, require that the coated material be attractive with good drape and handle, be water repellent, although not necessarily for prolonged use in heavy rain, and that the fabric retain these properties after dry cleaning or laundering.
There are several fabrics available that satisfy the conflicting requirements of waterproofness and breathability. One example is Entrant, which is a woven nylon fabric coated with a microporous polyurethane film formed by the so-called wet coagulation technique as described in U.S. Pat. No. 4,429,000 to Toray Industries, Inc. Other polyurethane-coated fabrics are described in U.S. Pat. No. 3,360,394 to Griffin. In the wet coagulation method a thin, microporous polyurethane layer is formed on a base fabric by applying a coating solution of a polyurethane dissolved in a polar organic solvent that will solubilize the polyurethane yet is miscible with water. The polymer solution is applied to the fabric substrate by knife coating or the like, then immersed in a bath of water which selectively dissolves or mixes with the organic solvent, exchanges water for the polar solvent and causes the previously dissolved polyurethane to coagulate leaving a thin, microporous coating having a cellular substrate on the fabric. Surface pores that result are generally one micron or less in diameter. Such pores are small enough to exclude water droplets and yet they provide a tortuous physical pathway from the base fabric to the coating surface, leading to a water-vapor-permeable fabric. The coating is a thin polymeric (polyurethane) film.
Polymeric materials such as plastics, foams and fibers can be protected from deterioration caused by light by the use of either ultraviolet absorbers or ultraviolet light stabilizers. The deterioration is usually manifested by yellowing, loss or change of color, or loss of desirable mechanical properties. The commonly-used ultraviolet absorbers, or screening agents, are often benzophenone derivatives, such as 2-hydroxybenzophenone. These reagents absorb ultraviolet light and re-emit the energy at another less harmful wavelength, or as heat. They usually suffer some degradation in the process of saving the polymer from degradation.
The object of this invention is protect thin microporous polyurethane films from deterioration caused by ultraviolet light. Topical treatments are known to provide a modest degree of ultraviolet (UV) protection for coated fabrics. These treatments are water-based in nature, and must be applied as a final finish by a final pad, dry, cure procedure. With most coated fabrics, this approach works, but with a microporous coating having a cell structure of many voids or micropores generally one micron or smaller in diameter, the pores are small enough to exclude liquid water. The UV-protective treatment therefore cannot penetrate the inner structure of the coating's matrix, leaving it unprotected, and it will yellow with aging or exposure to light.
Ultraviolet light stabilizers, of which hindered amine light stabilizers are a class, do not function by the typical mechanism of ultraviolet absorption, but rather interrupt the decomposition mechanism initiated by ultraviolet light. Tinuvin.RTM. 292 (registered trademark of Ciba-Geigy Corp, Ardsley, N.Y.), the stabilizer of choice in the present invention, is a hindered amine light stabilizer. Its structure and formula are given in the explanation of the invention that follows. Applicants have found that this product provides effective protection against ultraviolet light for 20 hours in AATCC Test Method 16A-1982. This is the level of protection required by the marketplace for apparel end uses. As shown by Table I, several ultraviolet absorbers were ineffective in providing the desired degree of protection. Our experience indicates that an ultraviolet light stabilizer, rather than an ultraviolet absorber, is required for use in microporous urethane foams.
Microporous coatings, as described above and elsewhere, are made by dissolving a polymer in a water-miscible solvent, then mechanically applying this solvent solution as a coating to a fabric. The thus-coated fabric is then immersed in a non-solvent, such as water. The microporous structure of the coating is completed when all the solvent has been displaced with the non-solvent, leaving a microporous layer on the base fabric.
Experience has made it clear that unprotected microporous coatings using aromatic polyester-based urethanes dissolved in N,N-dimethylformamide (DMF) have a severe light fastness problem and will not meet the apparel industry standard of effective protection against ultraviolet light for 20 hours.